Coated aluminum article and method of producing the same



Patented Mar. 3, 1936 UNITED STATES COATED AL UMIIIN'UM ABTHHE AND METHOD PRODUCING THE. SAME Robert B. Tanner, Detroit, Mich,

sang-nor to Metal Finishing Research Corporation, meta-cit,

Mich a corporation of Application Dctober 811, 1984i,

No Drawing.

Michigan serial do. 159,74!

it mains-e, (ill- H8 33 This lnventon relates to an article having a coated-aluminum surface and a method of pro= ducing that coating. More particularly, the invention relates to an article having an fiuminum surface coated with aluminum fluoride, and alumlnum fluoride produced in situ upon the sin-- face, and to a method of producing said coated surface. An article presenting a surface of aluminum or an aluminum alloy may be coated for m different reasons. The coating may be for purposes of resistance to corrosion, to facilitate coloring, as a foundation for paint or lacquer, to form electrical insulation, to aflect the absorption, transmission or radiation of heat or light, or for 35 other purposes.

By immersing the aluminum article in a weah solution of hydrofluoric acid substantially satu rated with aluminum fluoride, a coating of aluminum fluoride may be formed in situ upon the 26 aluminum surface. For example, in an aqueous solution containing 0.2% by weight of hydrofluoric acid and substantially saturated with aluminum fluoride, a very adherent coating can be formed at the boiling point in from 5 to 10 minutes. Such an aluminum fluoride coating is firmly adherent since it is formed substantially integrally with the subjacent metal. It also forms a base to which ordinary paint and lacquer adheres tenaciously, and in the event that the paint or lace quer becomes scratched at any point, the fluoride coating prevents any peeling ofl of the paint adiacent the scratch. Aluminum fluoride is very resistant, being substantially insoluble. not only in water, but in ordinary alkalies, acids and. alcohols. 5 Therefore, it forms a permanent coating upon the aluminum surface which resists corrosion and forms a very good base for paint, lacquer or other finishing coatings which may be applied to give the article any desired appearance. It also has 40 high electrical resistance, and modifies the conduction, absorption, reflection and emission of heat and like radiations.

While the solution of hydrofluoric acid does not form the fluoride coating until it has been sub- 5 stantially saturated with aluminum fluoride, it requires comparatively little almninum fluoride to sufllciently saturate the dilute solution and therefore the solution may be allowed to dissolve the necessary amount of aluminum from the article 50 when starting the coating operation unless a slight etching away of the article is unusually objectionable. In any case after a sufllciently dilute solution has been in use for a short time it becomes suiflciently saturated with aluminum fluoride so 55 that the subsequent action thereof merely produces the surface coating upon the aluminum subiected thereto. This applies, of course, only where a solution of the order of dilution indicated above is employed. With stronger solutions, an etching rather than a coating action re- 5 suits, as discussed below.

One easy way to substantially saturate a dilute solution sumciently for coating purposes is to allow the solution to act on aluminum powder or filings, but such a solution may be balanced, 10 when preferred, by adding aluminum fluoride as such.

With a solution containing 2% hydrofluoric acid the addition of about .6% aluminum fluoride balances the solution sufliciently to produce a good coating. Increased amounts of the fluoride make possible the production of a coating with somewhat increased acidity, a strength of .5% acid having been used with good results; but increase of acidity above this point is needless, introduces difliculties, and soon reaches a point where it is practically impossible to balance the solution with aluminum fluoride so as to give satisfactory results. with toomuch acid, or without the balancing salt even with very dilute acid, only pick- 5 ling results. If the balancing aluminum fluoride is added as such, an excess should be avoided, as a solution within a good coating range of dilution below 1% gives only a very thin coloration or iridescence rather an excess of aluminum fluoride is added.

Some other materials may be added to the solution for coloring or other purposes, and certain materials such as potassium permanganate, for example, may expedite the coating operation to some extent; but the coating operation may be easily inhibited by various foreign materials. There have been numerous proposals of solutions containing various amounts of hydrofluoric acid or compoimds thereof for-treatment of aluminum surfaces but so far as known these solutions have not been in such condition as to produce a coating of aluminum fluoride upon the surface of the aluminum. For example, the ordinary solution of hydrofluoric acid may be used to etch the surface of the aluminum, but as previously noted,

it does not form a coating upon that surface until the solution is substantially saturated with aluminum fluoride, after which it has a coating rather than an etching action. In other words, wherever hydrofluoric acid is used for etching purposes, it obviously is not used for coating. Also, as previously noted, the hydrofluoric acid I does not have a coating effect upon the aluminum in many combinations where other chemicals are 66 than a proper coating where 3 2 2,002,001 J v wesentwhichdohavesomeooatingeilectupon athickness iorthe twoooatingsoi' 0.0001 inch the aluminum. or one-hall that for a single coating. A coat- W'hiiethepreierredlolutionisaverydilute inghavingathicknessoithisorderisqmteetsolution or acid substantially sattective for modifying the surface of the aluminum fluoride formed in situ upon the surface of the i. A method of coating a metal surface. the

10 aluminum metal. major portion of which is metallic aluminum, 0

As indicated above. a coating formed in the which consists in immersing said surface in a manner specified forms a very :ood bonding hot dilute solution 01 hydrofluoric acid substanin; properties, modifies the absorption, trans- 2. An article having a metallic Surface, the

15 mission reflection and radiation oi heat. and is major portion of which consists 01' aluminum,

exceptionally resistant to corrosion. I! desired covered with a substantially integral co ting, the

rendered While the thickness of the coating may vary for measurement by microscopic means, 20 with conditions and duration of the coating proc- 3. An article having a metallic surface, the ass, that normally produced has a thickness major portion or which consists 01' aluminum.

2: sheets treated to render the edges or the coatorder of 0.00005 inc ings visible, measurement by Standard Bausch 0 Lamb microscopic measuring apparatus showed ROBERT R. TANNER. 

